The secondary battery, which is used in various fields, requires the appropriate charge/discharge control for the efficient operation thereof. This requires the remaining amount of the secondary battery (State of Charge; hereinafter simply referred to as an SOC) to be estimated with high accuracy.
Conventionally, a so-called current integration scheme is known, in which the input/output current of the secondary battery is integrated to estimate the SOC. In this current integration scheme, for example, in the case where the battery is not fully charged, the improvement in the estimation accuracy is limited due to the reasons including the effects of the error resulting from cancellation of the least significant bit in the processing device (CPU) performing the current integration, capacity decrease caused by self-discharge, and the like, or high dependence on the current sensor accuracy.
Thus, in order to improve the estimation accuracy, it is proposed that both of the electromotive voltage (Open Circuit Voltage; hereinafter simply referred to as an OCV) of the battery and the current integrated value are used to estimate the remaining amount of the battery (SOC) (for example, Japanese Patent Laying-Open No. 2003-149307). Specifically, the remaining-amount calculation method for the battery is proposed which is characterized in that an SOC correction parameter is determined by the electromotive voltage of the battery and the correction parameter is used to correct the SOC based on the current integration.
Similarly, as an approach for calculating the remaining amount using both of the current integration and the output voltage of the secondary battery, the remaining-amount control method is proposed for correcting, depending on the output voltage, a charge efficiency η used for the remaining-amount estimation based on the current integration (for example, Japanese Patent Laying-Open No. 2002-369391).
In particular, the secondary battery mounted in a hybrid vehicle and the like is subjected to charge/discharge control, based on the estimated SOC, so as to give a higher priority to charging or discharging assuming a control center value of the SOC as a boundary. Furthermore, upper and lower limit values are set in the SOC management range including the control center value to prevent excessive charging and excessive discharging. In other words, while charging is limited or inhibited in the case of exceeding the management upper limit value, discharging is limited or inhibited in the case of falling below the management lower limit value.
Therefore, in view of ensuring the safety in the charge/discharge control of the secondary battery, that is, avoiding excessive charging and excessive discharging of the secondary battery, it is desirable to estimate an SOC estimation value in the vicinity of the upper and lower limit values in the SOC management range with a higher degree of accuracy. Thus, the SOC correction parameter is set to be larger in the vicinity of the upper and lower limit values in the SOC management range.
However, the correction value is determined such that the estimated SOC, based on the integrated value of the charging/discharging current, approaches the estimated SOC based on the electromotive voltage. Consequently, even if the estimated SOC based on the current integrated value (for example, 85%) approaches the management upper limit value (for example, 90%) as compared to the estimated SOC based on the electromotive voltage (for example, 80%), the estimated SOC based on the current integrated value is largely corrected further downward, that is, to be away from the management upper limit value. In view of avoiding the excessive charging of the secondary battery, the above-described correction produces the opposite effect, which increases the possibility of excessive charging on the contrary.
Similarly, even if the estimated SOC based on the current integrated value (for example, 15%) is in close proximity to the management lower limit value (for example, 10%) as compare to the estimated SOC based on the electromotive voltage (for example, 20%), the estimated SOC based on the current integrated value is largely corrected further upward, that is, to be away from the management lower limit value, which increases the possibility of excessive discharging on the contrary.
Furthermore, the time change rate of the remaining amount of the secondary battery varies depending on the magnitude of the charging/discharging current of the secondary battery. Accordingly, in the case where the remaining amount is in proximity to the management upper limit value, the generation of large charging current may cause excessive charging. In the case where the remaining amount is in proximity to the management lower limit value, the generation of large discharging current may cause excessive discharging.